There exists a need for electronically steerable antennas capable of operating at large bandwidths, operating frequencies, and polarization diversity, with high radiated power levels. An active electronically scanned array (AESA) is an antenna architecture that provides such performance. The AESA is a phased array that integrates a transmitter-receiver module (TR module) at every radiating element. One attractive feature of an AESA is modularity of the design, such that a radiating aperture can be designed independently of a transmit-receive (TR) module. While radiating apertures having high efficiency, decades of bandwidth, and dual polarizations have been previously developed, such these arrays typically operate below 20 GHz, where unit cells are on the order of centimeters in size. At these length scales, arbitrary geometries can be fabricated with high precision using low cost printed circuit board (PCB) processing techniques. However, there is a need for AESAs that operate at mm-wave frequencies. Increasing the operating frequency enables higher communication data rates attributable to the increased absolute bandwidth. In addition, mm-wave radars can provide higher resolution images due to the reduced wavelength. However, reducing the operating wavelength from the centimeter to the millimeter regime introduces significant design challenges. In particular, the wavelength is only ˜10-100 times larger than the minimum feature size that can be fabricated using standard printed-circuit-board (PCB) or low temperature co-fired ceramic (LTCC) processing techniques, which limits the design freedom.
Operating at higher frequencies necessitates that the spacing between radiating elements should be scaled according to the wavelength. However, TR modules do not typically benefit from similar scaling laws, and TR modules providing roughly 1 W/element are typically several millimeters in size and independent of the frequency. At mm-wave frequencies, it is difficult to fit a 2D array of TR modules on a single printed-circuit-board (PCB). Another problem with such configurations is heat dissipation when operating at very high power levels.
Furthermore, it is desirable for the radiating aperture of a circularly polarized AESA to scan at wide angles of incidence from broadside. In this regard, linear-to-circular polarizers are used to convert an incident, linearly polarized plane wave into a transmitted, circularly polarized wave. Linear-to-circular polarizers are utilized from microwave to optical frequencies for a myriad of applications. Many of these applications also demand wide operating bandwidths and wide angles of incidence. However, conventional linear-to-circular polarizers only work perfectly at a single frequency making them inherently narrowband.
At THz frequencies and higher, wideband linear-to-circular polarizers are typically realized by cascading multiple birefringent waveplates with rotated principal axes. Polarizers utilizing cascaded waveplates can realize multiple octaves of bandwidth. At these higher frequencies, the geometry can afford to be many wavelengths in thickness while still maintaining a low profile since the wavelength is short. A disadvantage inherent in these designs is that they do not typically work well at wide angles of incidence since the optical thickness of the plate is a function of the angle of incidence.
At microwave frequencies, the most common linear-to-circular polarizers utilize cascaded patterned metallic sheets (i.e., sheet impedances) with subwavelength overall thicknesses. The bandwidth of microwave linear-to-circular polarizers are typically less than 40%. In some examples, the bandwidth has been increased up to an octave using meanderline metallic patterns printed on dielectric substrates. However, these meanderline polarizers do not typically work well at wide angles of incidence when their bandwidth is large.
Conventional waveplates composed of uniaxial dielectrics (i.e., εxx=εzz≠εyy) only operate at a single frequency. It has been known since the 1950's that the bandwidth can be significantly extended by cascading waveplates with different thicknesses and relative orientations to develop so-called achromatic waveplates. These waveplates are commercially available at optical frequencies with bandwidths of over 4:1. While this design approach has been scaled down from optical frequencies to THz and mm-waves, as the wavelength is increased further, the required thickness of naturally occurring crystals becomes prohibitive due to the notable, weight, size, and loss.
In view of the above, it would be advantageous to provide an AESA capable of operating effectively at high frequencies and very high power levels for radiating circular polarization.